1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a transflective thin film transistor substrate and method of fabricating the same.
2. Description of the Related Art
Liquid crystal display devices are generally classified into a transmissive type where a picture is displayed using light incident from a backlight unit, and a reflective type where a picture is displayed by reflecting external light such as a natural light. However, the power consumption of the backlight unit is high in the transmissive type, and the reflective type depends on the external light so that it cannot display a picture in a dark environment.
To resolve this problem, a transflective liquid crystal display device is increasingly being used, wherein the transflective liquid crystal can be selected to be in a transmissive mode where the backlight unit is used or in a reflective mode where the external light is used. The transflective liquid crystal display device operates in the reflective mode if the external light is sufficient and in the transmissive mode if the external light is not sufficient, thereby reducing the power consumption more than the transmissive liquid crystal display device but not being restricted by external light levels unlike the reflective liquid crystal display device.
Generally, a transflective liquid crystal display panel of the related art, as shown in FIG. 1, includes a color filter substrate and a thin film transistor substrate which are bonded together with a liquid crystal layer (not shown), and a backlight unit 60 arranged behind the thin film transistor substrate. Each pixel of the transflective liquid crystal display panel is divided into a reflective area where a reflective electrode 28 is formed, and a transmissive area where the reflective electrode 28 is not formed.
The color filter substrate includes an upper substrate 52, a black matrix (not shown), a color filter 54 formed on the upper substrate 52, a common electrode 56, and an alignment film (not shown) formed thereover. The thin film transistor substrate includes a lower substrate 2, a gate line 4, a data line (not shown) formed on the lower substrate 2 crossing the gate line 4 to define each pixel area, a thin film transistor connected to the gate line 4 and the data line, a pixel electrode 32 formed at the pixel area and connected to the thin film transistor; and a reflection electrode 28 formed at a reflection area of each pixel to overlap the pixel electrode.
The thin film transistor includes a gate electrode 6 connected to the gate line 4; a source electrode 16 connected to the data line; a drain electrode 18 facing the source electrode 16; an active layer 10 overlapping the gate electrode 6 with a gate insulating film 8 therebetween to form a channel between the source and drain electrodes 16 and 18; and an ohmic contact layer 12 to make an ohmic contact with the active layer 10, the source electrode 16, and the drain electrode 18. The thin film transistor responds to the scan signal of the gate line 4, thereby causing a video signal on the data line to be charged and maintained on the pixel electrode 32.
The reflection electrode 28 reflects an external light that is incident through a color filter substrate toward the color filter substrate. At this moment, the surface of an organic film 24 formed under the reflection electrode 28 has an embossing shape, and the reflection electrode 28 on top of the organic film 24 also has the embossing shape, thereby increasing its reflection efficiency due to its dispersion effect.
The pixel electrode 32 is connected via an upper storage electrode 20 to the drain electrode of the thin film transistor, and the pixel electrode 32 generates a potential difference with a common electrode 56 by the pixel signal supplied through the thin film transistor. The potential difference causes liquid crystal molecules having dielectric anisotropy to rotate, thereby controlling the transmissivity of the light that passes through a liquid crystal layer of each of the reflection area and a transmission area, and changing its brightness in accordance with the video signal.
In this case, a transmission hole 36 is formed in the relatively thick organic film 24 at a transmission area so that the length of the light path going through the liquid crystal layer is the same in the reflection area as in the transmission area. As a result, a path that ambient light incident at the reflection area, i.e., a reflection light RL, goes through the liquid crystal layer, then through the reflection electrode 28, and then through the liquid crystal layer in the liquid crystal layer is the same in length as a path that the transmission light TL of a backlight unit 60, which is incident at the transmission area going through the liquid crystal layer. Thus, the transmission efficiency becomes the same in both of the reflection mode and the transmission mode.
The thin film transistor substrate further includes a storage capacitor connected to the pixel electrode 32 to maintain the video signal supplied to the pixel electrode 32 stable. The storage capacitor is formed with an upper storage electrode 20 overlapping a storage line 40 with a gate insulating film 8 therebetween. Here, wherein the upper storage electrode 20 is extended from the drain electrode 18 to connect to the pixel electrode 32 via a contact hole 34. The ohmic contact layer 12 and the active layer 10 further overlap under the upper storage electrode 20 in the process.
The thin film transistor substrate further includes a first passivation film 22 between the thin film transistor and the organic film 24; a second passivation film 26 between the organic film 24 and the reflection electrode 28; and a third passivation film 30 between the reflection electrode 28 and the pixel electrode 32. Accordingly, the contact hole 34 penetrates the first to the third passivation films 22, 26 and 30, the organic film 24 and the reflection electrode 28 so that the pixel electrode 32 is connected to the upper storage electrode 20.
In such a transflective liquid crystal display panel, the thin film transistor substrate includes the semiconductor process and requires a plurality of mask processes. Thus, its manufacturing process is complicated so that it significantly increases the liquid crystal display panel manufacturing cost.
Hereinafter, a fabricating method of the transflective thin film transistor substrate according to the related art will be described in reference with FIGS. 2A to 2F. As shown in FIG. 2A, in a first mask process, a gate pattern including the gate line 4, the gate electrode 6, and the storage line 40 is formed on the lower substrate 2.
A gate metal layer is formed on the lower substrate 2 by a deposition method such as sputtering. Subsequently, the gate metal layer is patterned by a photolithography process using a first mask and an etching process, thereby forming the gate pattern including the gate line 4, the gate electrode 6, and the storage line 40. The gate metal layer is a single layered or double layered metal, such as Al, Mo, or Cr.
As shown in FIG. 2B, the gate insulating film 8 is formed on the substrate 2 having the gate pattern. On the substrate 2 having the gate insulating film 8, a semiconductor pattern having the active layer 10 and the ohmic contact layer 12 formed, and a source/drain pattern having the data line, the source electrode 16, the drain electrode 18 and the upper storage electrode 20 are stacked by the second mask process.
The gate insulating film 8, an amorphous silicon layer, an amorphous silicon layer with impurities doped thereto, and the source/drain metal layer are sequentially formed on the lower substrate 2 where the gate pattern is formed. The gate insulating film 8 is formed of an inorganic insulating material such as silicon oxide SiOx or silicon nitride SiNx, and the source/drain metal layer is the single layered or double layered structure of the metal such as Al, Mo or the like.
A photoresist pattern is formed on top of the source/drain metal layer by a photolithography process using a second mask. In this case, a diffractive exposure mask having a diffractive exposure portion at a channel of the thin film transistor is used as the second mask. Thus, the photoresist pattern of the channel has a lower height than the source/drain pattern portion. Subsequently, the source/drain metal layer is patterned by a wet etching process using the photoresist pattern to form the source/drain pattern that includes the data line, the source electrode 16, the drain electrode 18 integrated with the source electrode 16, and the storage electrode 20. Then, the amorphous silicon layer doped with the impurities and the amorphous silicon layer are simultaneously patterned by a dry etching process using the same photoresist pattern, thereby forming the ohmic contact layer 12 and the active layer 10. After removing the photoresist pattern having relatively low height at the channel by an ashing process, the source/drain pattern and the ohmic contact layer 12 of the channel are etched by a dry etching process. Accordingly, the active layer 10 of the channel is exposed to separate the source electrode 16 from the drain electrode 18. Subsequently, the photoresist pattern remaining on the source/drain pattern is removed by a stripping process.
As shown in FIG. 2C, a first passivation film 22 is formed on the gate insulating film 8 where the source/drain pattern is formed, and an organic film 24 is formed on top thereof by a third mask process. Here, the organic film 24 has a contact hole 34 and a transmission hole 36 with the embossing shaped surface.
The first passivation film 22 and the organic film 24 are sequentially formed on the gate insulating film 8 where the source/drain pattern is formed. The first passivation film 22 is formed of the same inorganic insulating material as the gate insulating film 8, and the organic film 24 is of a photosensitive organic material, such as an acrylic resin.
Then, the organic film 24 is patterned by a photolithography process using the third mask, thereby forming an open hole 35 and the transmission hole 36 which penetrate the organic film 24 in correspondence to the transmission portion of the third mask. At this moment, the third mask has a structure where a shielding portion and a diffractive exposure portion repeat at the rest area except for the transmission portion. The organic film 24 remaining in correspondence thereto is patterned to have a structure that a shielding area (projected portion) and a diffractive exposure area (groove portion) having a stepped difference are repeated. Subsequently, the organic film 24 where the projected portion and the groove portion are repeated is fired so that the surface of the organic film 24 has the embossing shape.
As shown in FIG. 2D, a second passivation film 26 is formed on the organic film 24 that has the embossing shape, and the reflection electrode 28 is formed on top thereof by a fourth mask process. The second passivation film 26 and the reflective metal layer are deposited to maintain their embossing shape on top of the organic film 24 that has the embossing surface. The second passivation film 26 is formed of an inorganic insulating material such as the first passivation film 22, and the reflective metal layer is formed of a metal such as AlNd or the like, of which the reflectivity is high. Subsequently, the reflective metal layer is patterned by a photolithography process using a fourth mask and the etching process to form the reflection electrode 28. Here, the reflection electrode is independent of every pixel and is opened at the transmission hole 36 and the open hole 35 of the organic film 24.
As shown in FIG. 2E, a third passivation film 30 covering the reflection electrode 28 is formed by a fifth mask process, and the contact hole 34 penetrating the first to third passivation films 22, 26, 30 is formed. The third passivation film 30 covering the reflection electrode 28 is formed and the contact hole 34 is formed by a photolithography process using a fifth mask and the etching process. Here, the contact hole 34 penetrates the first to third passivation films 22, 26, 30 at the open hole 35 of the organic film 24. The contact hole 34 exposes the drain electrode 18 and the upper storage electrode 20. The third passivation film 30 is formed of the same inorganic insulating material as the second passivation film 26.
As shown in FIG. 2F, a pixel electrode 32 is formed on the third passivation film 30 using a sixth mask process. A transparent conductive layer is formed on the third passivation film 30 by the deposition method such as sputtering, and the transparent conductive layer is patterned by a photolithography process using a sixth mask and the etching process to form the pixel electrode 32 at each pixel area. The pixel electrode 32 is connected to the upper storage electrode 20 through the contact hole 34. The transparent conductive layer is formed of indium-tin-oxide ITO.
In this way, the related art transflective thin film transistor substrate is formed by six mask processes, thereby complicating its manufacturing process is complicated.